The invention relates to a winding mechanism for roller blinds which are rolled onto a tube, of the kind in which a stationary rod inside the tube is held by an external fixture, one end of the tube being rotatably supported on the rod by means of a bearing sleeve, and in which a rotational force is created between the rod and the tube.
Such winding mechanisms are well known, and they have the following general features: The spring is tensioned when the blind is pulled down, and a latching device blocks the roller blind at desired adjustments. The construction provides an advantageous characteristic, because the force of the spring is at its maximum when the tube has to carry the largest length of hanging blind. When the latching device is freed by means of a sharp pull in the blind the roller blind is wound around the tube during a strong acceleration. When it has reached the top it has obtained the highest speed, and means have been provided to brake the bottom rail when it has been completely raised. It is important that the fully wound roller blind does not participate in the rotation, because this would relax the spring, so that the bottom rail would end dangling at an undefined height below the desired position. Because of this, the bottom rail is frequently stopped by means of its dimensions or by means of a projection fitted to the bottom rail. However, the sharp braking is harmful, both for the roller blind and seams provided in it, and for the window frame which receives beatings from the projections. It is under all circumstances desirable that the spring has a minimum bias when the roller blind is in its uppermost position.
Various braking devices have been tried to prevent the strong acceleration towards the fully wound condition, but a stationary friction must not be so large that the spring cannot overcome it by means of its least tension, and this means that the spring must be stronger and the action of the brake will be very dependent on a precise balancing of the remaining tension of the spring and the friction. A further problem arises in case the blind is made in a comparatively stiff fabric which has a memory effect relating to the position it has taken for some appreciable time. Hereby a force which would be suitable for winding from a particular starting position, would be unsuitable when starting from another starting position.
It has been suggested to use a centrifugal regulator in which weights are flung outwards due to the rotation of the tube and create friction, so that a speed-dependent braking is obtained. This is evidently desirable, however it is a solution which is mechanically very complicated, because the small rotational radius requires a high rotational speed to obtain a braking effect. One is required to use a gear train which can transform the rotational speed of the tube to a far higher rotational speed which inside a tube is usually performed by means of a multistage planetary drive. This calls for a large number of parts, some of which will rotate quickly which causes wear problems. Furthermore a unilateral clutch is required to disengage the brake and gear during lowering of the blind. This type of solution will not stop the blind in its uppermost position and prevent its relaxing the spring.
There is hence a need for a simple and stable solution which in connection with a roller blind may provide a controlled raising and stopping of the roller blind in its uppermost position which influences neither the roller blind itself nor its window frame surroundings.
This is obtained in a mechanism according to the invention which is particular in that a non-rotating threaded rod cooperates with a nut which is rotated by the tube during axial shifting, and in which an elastic axial force is created which at least during a fast movement in a first direction is increased due to the movement of the nut, and in which elements have been provided which establish a controlled friction against the rotation of the nut.
The axial force may be provided either by means of a helical spring or by means of a piston which acts on a volume of air in a cylinder.
It will have been noted that as the rod is non-rotating, a nut which is rotated will be simultaneously shifted axially. The rotation of the nut is e.g. created by means of a groove- and tounge connection between the inside of the tube and the outside of the nut, and the nut will move in a direction which either compresses the spring or relaxes it. The spring force basically establishes friction in three locations: 1) between the nut and the thread, 2) between one end of the spring and the nut, and 3) between the other end of the spring and a suitable shoulder. The well controlled friction may be created one of these places, unless particular elements are provided for this purpose, in which case the two locations last mentioned may be made into firm connections.
One embodiment is particular in that the elastic force is provided by means of a piston which acts against a closed volume of air in a cylinder. By providing a controlled exhaust of the air from the cylinder, any desired compression spring characteristic may be dynamically obtained, as it will have been noted that the greatest problem of braking occurs when the roller blind is fast rotating, whereas there is a need for sufficient spring force to obtain correct raising during slow rotation, where there is no stored energy due to the moment of inertia.
In this embodiment there are four possibilities of creating friction due to the axial spring: 1) between the nut and the thread, 2) between one end of the air cylinder spring and the nut, 3) between the other end of the air cylinder spring and a suitable shoulder, and 4) between the piston and the cylinder in the air cylinder spring. The well controlled friction may be created one of these places, unless particular elements are provided for this purpose, in which case the two locations last mentioned may be made into firm connections. The adjustment of the characteristic of the air cylinder spring, because it becomes dependent on whether the movement of the piston is able to build up a pressure which causes braking (fast rotation) in accordance with the calibration of an exhaust opening for air. During slow rotation, the air will be able to exhaust in time with its compression. Hereby the same advantages as those obtained in the mechanically complex centrifugal regulator are obtained in a simple manner. It would similarly be possible to change the spring characteristic as a function of the precise position of the roller blind, because several calibrated exhaust openings may be provided along the inner surface of the cylinder. As the air inside the cylinder is lost in use, a new volume is needed for each lowering, and it is hence expedient to provide an in-flow valve for the air during reversed axial movement.
One embodiment is particular in that the rod is divided into two parts by means of a separating shoulder, one of which is provided with a thread onto which is fitted a nut which is given a rotation during sideways movement by the tube, and in which a compression spring is provided between the shoulder and the nut. The place of friction 3 will be provided by the shoulder. This construction is also well adapted for the air cylinder spring discussed above.
An advantageous embodiment is particular in that the friction is established between the nut and the thread, the support of the compression spring on the nut, alternatively on the shoulder being reduced in friction. In this case an axial ball bearing or similar bearing may be provided between the spring and one of its supports. This construction is equally well adapted for use with the air cylinder spring discussed above.
A further advantageous embodiment is particular in that the friction is established by means of the support of the compression spring on the nut, alternatively on the shoulder, the connection between the thread and the nut being reduced in friction. In this case the preparation of one end of the spring or a suitable shoe ensures the well controlled friction. In this case the thread and nut may be made in low-friction materials, or a nut with recirculating balls may be used. This construction is equally well adapted for use with the air cylinder spring discussed above.
In order to prevent that the nut wedges itself into a lock at the remote end (with the spring expanded) during braking, care is taken that the rotation is stopped while the thread still has a controlled or negligible friction. This is advantageously obtained in that the nut is provided with an abutment at the periphery for interacting with a further shoulder at the end of the thread which stops the rotation of the nut before its movement is blocked by hitting the shoulder axially. It is hence a rotating and not an axial movement which performs the stopping. This blocked position corresponds to the uppermost position of the roller blind and is adjusted during fitting. This construction is particularly useful in conjunction with the use of an air cylinder spring.
In a further advantageous embodiment the support of the compression spring on the shoulder has a reduced friction in that the shoulder is constituted of the bearing sleeve of the rod, the compression spring being supported between the sleeve and the nut. In this case the compression spring is completely surrounding also the raising spring, and it must hence be considerably longer. Thereby a bearing with reduced friction is completely eliminated, because the spring is now only subjected to axial forces, it follows the rotation, because it is supported between the bearing which at one end rotates around the non-rotating rod and the nut which follows the rotation. Thereby there will also be a smaller variation in the force of the spring because of reduced percentage-wise variation in the length of the compression spring.
A further embodiment is particular in that the helical compression spring is supported between a plug with a bearing journal and the nut.
A further embodiment further develops this solution and is particular in that the threaded rod is taken through the plug and is fixed against rotation by means of a fixture independent of the clamping of the rod for the raising spring. This means that the fixture at the opposite end of the roller blind, which traditionally has a bearing for a journal, will be made to hold the threaded rod against rotation in similarity with the fixture discussed in the introduction of the description.